When a hair treatment such as perm, coloring and the like is applied, it is important to increase hydrophilicity of protein constituting the hair, namely, to swell the hair, so as to enhance permeation of a drug solution to the hair. In this way, permeation of chemicals such as a perming agent, a hair color and the like is facilitated. A hair swelling treatment is mostly performed using an alkaline substance such as ammonia, monoethanolamine and the like (e.g., JP-A-1-503064). By doing so, the alkaline substance acts on the hair to ionize polar groups in protein in the hair, and induces structural changes. However, such hair treatment using an alkaline substance simultaneously causes side effects such as hydrolysis of protein, loss of lipid and the like in the hair, and consequently damages hair and induces skin irritation.
On the other hand, it is important to permeate an active ingredient contained in a cosmetic or a skin external preparation to hair or skin and enhance the permeation, thereby to increase the effect of the active ingredient. For this end, various permeation enhancers have conventionally been used. As such permeation enhancer, dimethyl sulfoxide, oleic acid, benzyl alcohol and the like are known (e.g., JP-A-8-500365). However, such permeation enhancers simultaneously have problems of skin irritation and the like, and are insufficient in terms of safety. Also, the sensory feel thereof is not good. Therefore, conventional permeation enhancers are not sufficiently satisfactory for users.